1. Field of the Invention
The present invention relates to the field of impedance matching networks. More specifically, the present invention relates to a method and apparatus for matching the impedance of a load with the internal impedance of a radio frequency (RF) power generator to provide maximum power transfer, where the frequency of the applied voltage generated by the RF power generator and the impedance of the load may independently vary.
2. Description of the Related Art
The Federal Communications Commission (FCC) has designated Industrial, Scientific and Medical (ISM) frequencies at 13.56 MHz, 27.12 MHz and 40.68 MHz, respectively. ISM frequencies may be radiated by equipment into the atmosphere without concern for causing radio frequency disturbances to other equipment. Plasma etch and deposition equipment manufacturers have traditionally used the 13.56 MHz frequency to operate a plasma chamber for manufacturing integrated circuits and plasma displays. However, ISM frequencies do not always provide an optimum frequency at which to operate a plasma chamber to achieve critical process steps, especially in view of decreasing integrated circuit dimensions. As a result, equipment manufacturers have developed plasma chambers that are capable of operating over a range of frequencies.
ISM-based RF power generators, however, are commonly designed to assure minimum deviation from a set ISM frequency, e.g., 13.56 MHz. In contrast, a variable frequency RF power generator ("generator") may be coupled to a load, e.g., a plasma chamber, to manipulate the frequency of the voltage applied to the load so that the load may be operated over a range of voltage frequencies. However, in an alternating current (AC) circuit, impedance is affected by the frequency of the applied voltage, which impedance, in turn, affects the transfer of power between the generator and the load. Moreover, the impedance of a plasma chamber may vary independent of the frequency of the applied voltage depending on such variables as chamber pressure, gas composition, and plasma ignition. What is needed, therefore, is an impedance matching network that allows the frequency of the applied voltage to vary while maintaining the impedance of the load with respect to the generator, i.e., the input impedance to the generator.
As is well known to those of ordinary skill in the related art, impedance for a given circuit may be comprised of both a resistive component and a reactive component, the latter of which may be either inductive or capacitive. Maximum power transfer bet,ween a generator and an attached load is achieved when the resistance of the load is equal to the internal resistance of the generator and the net reactance between the load and generator is zero. Thus, it is advantageous to counterbalance the reactance between the generator and the load to achieve a net reactance of zero. A net reactance of zero between the generator and load occurs when the impedance of the load is the conjugate of the internal impedance of the generator. Thus, if the generator has an inductive reactance, then a load that has a capacitive reactance of equal magnitude and opposite phase will result in a net reactance of zero to the circuit comprising the generator and the load, and vice versa. An impedance matching network may be utilized to maintain an input impedance that is the conjugate of the internal impedance of the generator as the frequency of the voltage applied by the generator to the load varies, and/or as the impedance of the load varies, so that maximum power transfer occurs between the generator and the load.
With reference to FIG. 1, a prior art impedance matching network 100 is illustrated. An RF power generator can be coupled to RF input 120. A load such as a plasma chamber can be coupled to RF output 130. The impedance matching network 100 ("network") comprises a phase detector 101 that samples the transmission line 108 at a fixed impedance, e.g., 50 ohms, and generates a signal over line 112 to control board 110. Control board 110 then causes servo motor 107 to turn variable capacitor 106, depending on the polarity of the phase shift between the input RF voltage and current caused by a non-linear impedance in the load, e.g., as occurs under ignited plasma conditions in a plasma chamber.
Magnitude detector 109 also samples the deviation from an impedance of, e.g., 50 ohms, on transmission line 108, and generates a signal over line 111 to control board 110 based thereon. Control board 110 then causes servo motor 103 to turn variable capacitor 102. The capacitance provided by capacitor 102 is also dependent, to a lesser extent, on the polarity of the phase shift between the RF voltage and current. The magnitude detector 109 detects the deviation from a characteristic impedance of, for example, 50 ohms. If the impedance in line 108 is greater than 50 ohms, the signal transmitted over line 111 is positive, and if the impedance in line 108 is less than 50 ohms, the signal transmitted over line 111 is negative. As can be seen, the prior art impedance matching network 100 is relatively slow because of the time needed for servo motors 103 and 107 to turn capacitors 102 and 106, respectively, to match the impedance of the generator with the impedance of the load. Moreover, the network 100 does not allow the generator to change the frequency of the applied voltage as may be desired depending on the load.
Today, semiconductor and flat panel plasma display equipment manufacturing process times are decreasing, such that the amount of time required to establish matching impedance between an RF power generator and a plasma chamber (whose operating frequency and impedance varies) is a limiting factor affecting throughput on the manufacturing line. What is needed is an impedance matching network coupling a generator to a load, e.g., a plasma chamber, that allows the generator to vary the frequency of the voltage applied to the load and utilizes fixed solid state components to rapidly and accurately adjust the input impedance of the attached load to maintain maximum power transfer to the load.